Hybrid Plasmonic Nanostructures May Improve Spectroscopic Techniques

Daniel S. Burgess

Nanorice, tiny metallic particles shaped like grains of rice, combines the plasmonic properties of nanorods and nanoshells to enable its application in surface plasmon resonance and surface-enhanced Raman spectroscopies for molecular sensing.

The grains of “nanorice” feature a hematite core and a gold shell. By controlling the thickness of the shell, the researchers can tune the plasmonic nanostructures to respond to the desired optical wavelength for sensing and imaging applications. Courtesy of Hui Wang, Rice University.

Developed by scientists at Rice University’s Laboratory for Nanophotonics in Houston, nanorice exhibits local field enhancements that are several times larger than that from nanoscale bow-tie junctions (see “Nanoscale Bow Ties Act as Optical Antennas,” Photonics Spectra, April 2006, page 91) and that are similar to that from nanoshell junctions. It further displays the highest surface plasmon resonance sensitivity of any metal nanostructure thus far reported.

Nanoscale metallic particles enhance local electromagnetic fields through the action of plasmons, optical excitations coupled with oscillations of their conduction electrons. The phenomenon enables novel sub-diffraction-limit amplification and focusing effects for sensing and imaging applications.

Naomi J. Halas, director of the laboratory, explained that the highest field enhancements have been obtained at the junction of two nanoshells — spherical structures that feature a dielectric core and a metallic shell. Nanoshells are promising for sensing applications because their optical resonance can be tuned by changing the relative size of the core and shell, but it would be more convenient to employ particles such as nanorods that display strong field enhancements at their easily accessible tips.

The investigators thus set out to create hybrid structures with the best characteristics of nanoshells and nanorods. The resulting nanorice features a nanorodlike hematite core and a gold shell.

To produce the 340 × 54-nm cores, they use forced hydrolysis of an aqueous solution of ferric chloride. They functionalize the cores with an organosilane for the attachment of 2-nm-diameter gold spheres capped with tetrakis hydroxymethyl phosphonium chloride, which act as nucleation sites for the electroless plating of the spindles in chloroauric acid and formaldehyde. The ratio of seed particles to AuCl^1–₄ ions in the reaction determines the thickness of the resulting gold shell and, thus, the plasmonic properties of the nanorice.

They calculate that nanorice displays a field enhancement of more than 7000, similar in magnitude to the “hot spot” at a nanoshell junction but located at the tips of a grain so that the site is more accessible and the enhancement extends tens of nanometers into the environment. Experiments involving surface plasmon resonance in various solvents revealed that the sensitivity of the longitudinal plasmon resonance wavelength is as high as 801.4 nm per refractive index unit and is relatively independent of the thickness of the shell.

The nanorice displays the field enhancements of nanoshell junctions, but at the exposed ends of the structures.

The next step for the researchers is to put the nanorice into use.

“Now that we understand the underlying physical principles behind these new nanoparticles, we can design them specifically for applications ranging from biomedical diagnostics and therapeutics to chemical sensing to new active, optically responsive nanodevices,” Halas said.

Nano Letters, online March 11, 2006, doi:10.1021/nl060209w.